Lanthanide Color Origin

The color observed in lanthanide ions ($Ln^{3+}$) in aqueous solutions arises from electronic transitions between different energy levels within their partially filled $4f$ orbitals. These are known as $f$-$f$ transitions. Ions with empty ($f^0$), half-filled ($f^7$), or completely filled ($f^{14}$) $f$ subshells generally do not exhibit color because these transitions are either impossible or very weak.

Ion Electronic Configurations and Color

Let's examine the electron configurations of the given lanthanide ions:

• $La^{3+}$: Lanthanum ($La$) has atomic number 57. $La^{3+}$ has the configuration $4f^0$. With no electrons in the $f$ subshell, it cannot exhibit $f$-$f$ transitions and is colorless.
• $Eu^{3+}$: Europium ($Eu$) has atomic number 63. $Eu^{3+}$ has the configuration $4f^6$. The $f$ subshell is partially filled, allowing for $f$-$f$ transitions, resulting in color (typically pink or red in solution).
• $Gd^{3+}$: Gadolinium ($Gd$) has atomic number 64. $Gd^{3+}$ has the configuration $4f^7$. Although the $f$ subshell is half-filled and contains unpaired electrons, the transitions in $Gd^{3+}$ are often very weak, leading to ions that are essentially colorless or very faintly colored in solution.
• $Lu^{3+}$: Lutetium ($Lu$) has atomic number 71. $Lu^{3+}$ has the configuration $4f^{14}$. The $f$ subshell is completely filled. Similar to $La^{3+}$, it lacks the possibility for $f$-$f$ transitions and is colorless.

Conclusion on Colored Ion

Based on the electronic configurations, $Eu^{3+}$ is the lanthanide ion among the choices that exhibits color in aqueous solution due to its partially filled $4f$ subshell ($4f^6$).